The present invention relates to data communication apparatus for data communication via cables and methods for the data communication
Known data communication apparatus will be described using a serial interface based on IEEE Std 1394a-2000 for carrying out communication using a differential signal. FIGS. 8A and 8B respectively show known data communication apparatus 200 and connection configurations thereof.
In FIG. 8A, the data communication apparatus 200 is constituted by a first communication circuit 201, a second communication circuit 202 and a control circuit 203. The first communication circuit 201 is coupled to the second communication circuit 202 of the data communication apparatus 200 at the other end of the communication via a cable 210, thereby carrying out data communication.
FIG. 8B shows respective configurations of the first communication circuit 201 and the second communication circuit 202. The first communication circuit 201 includes: a common-mode-potential setting circuit 220 for setting a common-mode potential of a differential signal for use in data communication; a driver circuit 221 for transmitting data; and a receiver circuit 222 for receiving data. The second communication circuit 202 includes: a common-potential detecting circuit 223 for detecting the common potential; a driver circuit 224; and a receiver circuit 225.
Now, it will be described how the data communication apparatus 200 operates.
First, when the first communication circuit 201 is ready for communication, the common-mode-potential setting circuit 220 outputs a common-mode potential of a differential signal. This common-mode potential is detected by the common-mode-potential detecting circuit 223 of the second communication circuit 202, thereby initiating communication. Data transmitted from the driver circuit 221 of the first communication circuit 201 is received by the receiver circuit 225 of the second communication circuit 202, whereas data transmitted from the driver circuit 224 of the second communication circuit 202 is received by the receiver 222 of the first communication circuit 201. In this manner, bidirectional communication can be performed.
In the data communication apparatus 200, when application of a power supply voltage Vcc to the first communication circuit 201 is stopped, the functions of the driver circuit 221 and the receiver circuit 222 in the first communication circuit 201 stop, but the second communication circuit 202 continues transmission and reception, resulting in a malfunction occurring in the second communication circuit 202. The control circuit 203 in the data communication apparatus 200 detects this malfunction, thereby stopping communication between two pieces of communication apparatus 200 and 200. In the case where power supply to the second communication circuit 202 is stopped, the same operation as described above is performed. Such a technique is disclosed in Japanese Laid-Open Publication No. 2-128533, for example.
However, the known data communication apparatus 200 has a drawback because of the following configuration. In a period between a stop of power supply to one of the two pieces of data communication apparatus 200 and a stop of the communication, the second communication circuit 202 of the other piece of data communication apparatus 200 continues transmission and reception, and a malfunction occurs. Then, the stop of power supply is detected by detection of this malfunction. Therefore, in the known data communication apparatus 200, a malfunction in communication always occurs.
If the power supply to the control circuit 203 is stopped at the same time as a stop of power supply to the data communication apparatus 200, there arises a problem that it is difficult to detect a malfunction with the control circuit 203.